This application claims the benefit of International Application Number PCT/SE00/02152, which was published in English on May 17, 2001.
The present invention generally relates to moulding and specifically to a method and an apparatus, respectively, for gas assisted moulding.
Moulding is a manufacturing technique, wherein a polymer is injected into a moulding tool under pressure and solidifies. The moulding tool is substantially a negative of the detail which is to be manufactured. Conventional injection pressures vary from 5000 to 20000 psi. Since these high pressures occur, the moulding tool, which often consists of two mould halves, must be held in a clamped state during injection and cooling. This clamping force needs to be considerably high.
The moulding technique can be used for the manufacturing of a large number of details with a very high precision. Tolerances better than 0.0025 mm are easily achieved by using a suitable combination of mould design, material and detail design. Further, moulding is a high-capacity process. Cycle times vary from a few seconds to several minutes in dependence on the size and form of the moulded detail.
The moulding tools are relatively expensive and they have to be designed to offer the high precision and at the same time be sufficiently robust to withstand the high pressures that occur. They may be fabricated in aluminium but are preferably fabricated in hardened tool steel in order to be usable during a long time. Thus, the moulding technique is particularly preferable when a large number of details are to be manufactured.
Gas assisted moulding (also called GAM, gas assisted injection moulding, or GID, Gasinnendruck), which is an improvement of the moulding technique and which relatively recently has been commercialized, allows sections of the interior of a product to be removed or, simply, that hollow products can be manufactured. The technique, in the following denoted gas assisted moulding, is suitable for the manufacturing of thicker products, such as handles, as well as products having thinner walls.
The technique comprises that a moulded detail is pressurized with a gas via a network of flow channels or directly in the moulded detail prior to allowing the injected material to solidify. The pressurized gas provides the packing force required for the manufacturing of a hollow moulded quality detail.
An overview of gas assisted moulding including discussions of the technique as well as applications is found in xe2x80x9cGas Assisted Mouldingxe2x80x9d, T. C. Pearson, Rapra Review Report, No. 103, 1998. In the overview the choice of equipment is discussed, including e.g. positioning and time adjustments of gas injection, and dimensions and positioning of gas channels.
Advantages of gas assisted moulding includes that less material is consumed to manufacture the detail (up to 45% less consumption), that better dimension stability can be achieved, that shrinkage marks, i.e. hot spots, can be eliminated, that details having higher strength and rigidity can be manufactured, that cycle times can be reduced, that moulding induced stresses in the material of the detail can be heavily reduced, and that a lower clamping force is required.
The use of a pressurized gas for assistance during conventional moulding of polymer is believed to have been commercially applicable by means of an invention by Friederich, which was patented 1978, see U.S. Pat. No. 4,101,617. The invention relates to moulding of hollow articles in a single step, wherein a compressed gas is introduced together with, or directly after, the injection of molten polymer in the article-defining mould.
Further, different particular aspects of gas assisted moulding have been patented during later years, see e.g. the patents U.S. Pat. No. 5,728,329; U.S. Pat. No. 5,662,841; U.S. Pat. No. 5,558,824; U.S. Pat. No. 5,705,201; U.S. Pat. No. 5,411,685; U.S. Pat. No. 5,110,553; U.S. Pat. No. 5,069,858; and U.S. Pat. No. 5,204,050; and references therein.
The three first mentioned of these patents relate to the pressurizing of a gas in the mould prior to injection of the molten polymer therein and generation of a static pressure with the molten polymer. The third one particularly depicts a technique for controlling the exhaust of the initial gas, whereby the flow front and the expansion velocity of the injected molten polymer are modulated in order to achieve a uniform material flow in the mould to minimize marks due to non-uniform flow in order to ensure that no gas blows occur in the material flow and to achieve articles having a more uniform wall thickness.
Further, the fourth patent discloses a vibration-based process to modify the injection moulding and/or the properties of the mould material.
The fifth patent depicts a gas control unit for a gas assisted moulding system, wherein an amount of gas is introduced in the mould in combination with an amount of polymer material during injection moulding.
The sixth and the seventh patents show process methods to enhance the surface quality of the manufactured articles.
The first mentioned of these patents comprises first to inject a considerable amount of polymer in the mould and thereafter to simultaneously inject pressurized gas and a further amount of polymer. The introduction of the pressurized gas prevents first that the first polymer flow is stopped and thereafter, subsequent to the introduction of the additional amount of polymer, the gas distributes the total amount of molten polymer in the article-defining mould. Particular ranges for the gas pressure and for the mutual relation between the two polymer amounts are given.
The latter of these patents depicts a moulding process, wherein a first amount of pressurized gas is assisting during moulding, but does not enter the article-defining mould, but enters in a volume substantially adjacent to the mould in order to assist at the filling out of the mould.
Finally, the eighth patent depicts a method and a device, respectively, wherein an article is manufactured by injecting molten polymer into a mould and by injecting an amount of pressurized gas in the polymer in order to fill out the mould and create a cavity in the polymer. Thereafter gas is injected into the polymer in the mould at a second position, which either forms a separate cavity in the polymer or which forms a passage through the polymer into the former cavity. The first gas is preferably injected at the same position as the polymer is injected and the latter gas can be injected at a position having a direct communication with the mould. The gas is vented when the article has solidified enough such that occurring valves, discharge outlets or the like will not be clogged. This venting may be performed from either one of the injection positions or from both, simultaneously or in some sequential order. This technique exhibits advantages comprising i.a. short cycle times, minimization of operation stops, low material consumption and manufacturing of articles of high quality.
The present invention relates to a further enhanced technique during manufacturing by gas assisted moulding, which exhibits the advantage of the above mentioned technique but which also simultaneously provides for further reduced cycle times.
Thus, it is a main object of the present invention to provide a method for effective, fast and reliable gas assisted moulding having considerably shortened cycle times.
A further object of the invention is to provide a method, which provides for manufacturing of high-quality products having high strength and rigidity without any shrinkage marks or moulding induced stresses.
Still a further object of the invention is to provide a method for moulding of products, wherein the consumption of primary material is low.
Yet a further object of the invention is to provide a method, wherein the cooling of the moulded product can be controlled.
Still a further object of the invention is to provide an apparatus, wherein said method for moulding can be implemented.
These and other objects of the invention are attained by a method and an apparatus according to the appended patent claims.
An advantage of the invention is that the cycle times can be considerably reduced compared to prior art techniques, possibly by at least up to 30%.
Further advantages of the invention and characteristics thereof will be apparent from the following description.